This invention relates to a rotary bushing for guiding a drill string of a blast hole drill, and more particularly to a bushing that is mountable in the deck or table of the blast hole drill and through which drill pipe passes for drilling into the earth.
The drill string of a conventional blast hole drill extends downwardly through an opening in the deck of a drilling machine. A tubular cylindrical guide bushing is mounted on the deck through which the drill string passes with a relatively close fit, the bushing serving to guide the drill string and also to provide a bearing and wear surface. Such deck bushings have heretofore generally been rigidly mounted with respect to the deck, as a result of which lateral vibrations emanating from the drill string are transmitted directly to the drilling machine.
Very substantial lateral vibrations can occur in a drill string, for example when the bit encounters hard rocks or other irregularities. As the drill string becomes longer, there can be very significant column bending of the string, and this can result in additional or magnified vibrations. Also, resonant conditions can occur at certain lengths and/or rotational speeds, again resulting in added or magnified lateral vibration. Where the bushing is rigid with respect to the deck, lateral vibrations are transmitted directly to the drilling machine and serious structural damage can result; there are many instances where vibrations have resulted in broken welds and other structural damage to the machine. Further, undampened vibrations cause excessive wear of the drill string sections or joints and drill bits, or result in wear or damage to the rotary driving head.
There are presently known rotary bearings which are mounted between the deck and drill pipe and consist of a metallic inner collar which rotates with the pipe and an outer collar fixed to the deck with ball bearings disposed in between. See, for example, U.S. Pat. No. 3,951,470. Such rotary bushings will reduce the wear on the drill string, but they are very expensive. Other pipe guides such as are shown in U.S. Pat. No. 3,194,611 utilize a plurality of rollers in the bushing which are encased with the pipe. See also U.S. Pat. Nos. 4,054,332; 4,076,338; and 4,326,756. Other patents teach a guide bushing having an elastomeric member or segments disposed within a collar and circumscribing the drill pipe. See, for example, U.S. Pat. Nos. 3,944,300 and 4,324,438. The guide bushings which provide rubber rings or collars often have the rings or collars bonded to their exterior surfaces. These may successfully dampen vibrations, but only for a very short time since they simply do not have the requisite mechanical strength. A particular problem in this regard is that the drill string also exerts rotational and vertical forces on the bushing, and these, with the lateral movements, result in a grinding action which can destroy the rubber rings or collars in a short period of time. Making the rubber harder may increase its life, but it also reduces its effectiveness in dampening vibrations. Further, the resilience of such bushings cannot be adjusted to meet varying conditions.
Guide bushings made entirely of metal are extremely noisy in operation and transmit excessive vibrations to the blast hole drill. Such metal bushings do not have a long service life and tend to rapidly wear the drill string.
U.S. Pat. No. 4,186,973 discloses a guide bushing mounted in a supporting sleeve in the deck of a blast hole drill for guiding a drill string through the sleeve. The bushing has a tubular core with a cylindrical axial bore to receive and guide the drill string. The core has a flange which is supported on the supporting sleeve, and there is a resilient, elastomeric cushion means mounted on the exterior of the core to serve as a cushion to soften and transfer movements of the bushing within the supporting sleeve. However, the tubular core is relatively stationary with respect to the supporting sleeve and does not rotate with the drill string, thereby causing excessive wear to the core.
The present invention is seen to have overcome these deficiencies of the prior act.